1. Field of the Invention
The present invention relates to a tire inflation pressure monitoring apparatus for checking the inflation pressure of pneumatic tires of, for example, vehicles, aircraft, etc.
2. Description of the Related Art
Described in Jpn. Pat. Appln. KOKOKU Publication No. 4-40211 (U.S. Pat. No. 4,742,857), U.S. Pat. No. 4,953,394, etc. are tire inflation pressure monitoring apparatuses which are proposed as means for detecting the inflation pressure of the tires of a vehicle while the vehicle is running. In these monitoring apparatuses, a transducer including a movable magnet is attached to a wheel rotating integrally with a tire, and a magnetic sensor is attached to a member on the vehicle body side. The movable magnet is designed so as to shift its position in the axial direction of the wheel as the tire inflation pressure changes. Thus, the higher the inflation pressure, the shorter the minimum distance from the movable magnet to the magnetic sensor is.
The magnetic sensor, which includes a magnetic coil, is used to detect the magnetism of the magnet as the wheel rotates, and generates an electromotive force of a magnitude corresponding to the minimum distance from the magnet by magnetic induction. This electromotive force is converted into a digital integral by means of an integrator and analog-to-digital converter. Hereinafter, this value will be referred to as A/D integral.
The movable magnet moves for a distance proportional to the tire inflation pressure in the axial direction of the wheel. If the position of the movable magnet with the tire at a reference pressure and the changing rate of the position of the movable magnet, compared to the change in pressure, are obtained in advance, therefore, the pressure can be obtained by detecting the moved distance of the magnet by using some suitable means (e.g., magnetic sensor). Theoretically, therefore, it is necessary only that the change of the minimum distance from the magnetic sensor to the movable magnet be computed in accordance with the variation of the A/D integral of the aforesaid induced electromotive force so that the tire inflation pressure can be obtained on the basis of the change in distance.
Actually, however, the wheels of a running vehicle roll in some measure as they rotate. As the vehicle corners, moreover, the wheels are subjected to a lateral load, so that they are distorted or axially moved with respect to the vehicle body. Inevitably, therefore, the distance from the magnetic sensor to each wheel varies. Thus, the minimum distance from the magnetic sensor to the magnet may be changed by some external factors irrelevant to the tire inflation pressure, in some cases. In consequence, the tire inflation pressure cannot be accurately determined by only measuring the magnetic sensor output by means of the movable magnet.